List of solar cycles

The following is a list of solar cycles (sometimes called sunspot cycles), tracked since 1755 following the original numbering proposed by Rudolf Wolf in the mid-19th century. The source data are the revised International Sunspot Numbers (ISN v2.0), as available at SILSO.[1] Sunspot counts exist since 1610[2] but the cycle numbering is not well defined during the Maunder minimum.[3] It was proposed that one cycle might have been lost in the late 18th century,[4] but this still remains not fully confirmed.

The smoothing is done using the traditional SIDC smoothing algorithm.[5] Using this algorithm, if the month in question is notated month 0, a weighted average is formed of months -6 to 6, where months -5 to 5 are given weightings of 1, and months -6 and 6 are given weightings of 0.5. Other smoothing formulas exist, and they usually give slightly different values for the amplitude and timings of the solar cycles. An example is the Meeus smoothing formula,[6] with related solar cycles characteristics available in this STCE news item.[7]

The start of solar cycle 25 (as with any other cycle) will be determined restrospectively. Technically, due to the calculation method, this declaration cannot happen less than 7 months after the minimum, although it can often be seen earlier that at least a temporary minimum will occur. Realistically, the declaration must happen some time after this, to discount the possibility of a double trough. When the start of cycle 25 is retrospectively announced, a large number of spotless days that were sitting in cycle 24 will be transferred to cycle 25.

It is more than likely that cycle 25 will begin in December 2019. As at August 12 2020, the way the SIDC algorithm works, even if there are no more spots before the end of March 2021, the averages for the first nine months of 2020 cannot fall below the December 2019 average. If there is no second trough, this December minimum will be 1.81. This would make cycle 24 the only "11-year solar cycle" to have lasted precisely 11 years. The value for January 2020 is 2.2. Here are the minimum possible values for subsequent months (as at August 12 2020):

February 2020: 2.7 (If no more sunspots before Aug 31)

March 2020: 2.8 (If no more sunspots before Sep 30)

April 2020: 2.8 (If no more sunspots before Oct 31)

May 2020: 2.7 (If no more sunspots before Nov 30)

June 2020: 2.6 (If no more sunspots before Dec 31)

July 2020: 2.3 (If no more sunspots before Jan 31 2021)

Aug 2020: 2.0 (If no more sunspots before Feb 28 2021)

Sep 2020: 2.0 (If no more sunspots before Mar 31 2021)

Oct 2020: 1.7 (If no more sunspots before Apr 30 2021)


August 12 2020: The current streak of 23 consecutive days with sunspots is the longest since a streak of 69 days ending October 2017, beating two streaks of 15 days in 2018.


Solar Cycle Start (Minimum) Smoothed minimum ISN (start of cycle) Maximum Smoothed maximum ISN Average spots per day Time of Rise (years) Duration (years) Spotless days[8][9]
Solar cycle 1 1755-02 14.0 1761-06 144 6.3 11.3
Solar cycle 2 1766-06 18.6 1769-09 193 3.3 9.0
Solar cycle 3 1775-06 12.0 1778-05 264 2.9 9.3
Solar cycle 4 1784-09 15.9 1788-02 235 3.4 13.6
Solar cycle 5 1798-04 5.3 1805-02 82 6.8 12.3
Solar cycle 6 1810-08 0.0 1816-05 81 5.8 12.8
Solar cycle 7 1823-05 0.2 1829-11 119 6.5 10.5
Solar cycle 8 1833-11 12.2 1837-03 245 3.3 9.7
Solar cycle 9 1843-07 17.6 1848-02 220 4.6 12.4
Solar cycle 10 1855-12 6.0 1860-02 186 92 4.2 11.3 561
Solar cycle 11 1867-03 9.9 1870-08 234 89 3.4 11.8 942
Solar cycle 12 1878-12 3.7 1883-12 124 57 5.0 11.3 872
Solar cycle 13 1890-03 8.3 1894-01 147 65 3.8 11.8 782
Solar cycle 14 1902-01 4.5 1906-02 107 54 4.1 11.5 1007
Solar cycle 15 1913-07 2.5 1917-08 176 73 4.1 10.1 640
Solar cycle 16 1923-08 9.4 1928-04 130 68 4.7 10.1 514
Solar cycle 17 1933-09 5.8 1937-04 199 96 3.6 10.4 384
Solar cycle 18 1944-02 12.9 1947-05 219 109 3.3 10.2 382
Solar cycle 19 1954-04 5.1 1958-03 285 129 3.9 10.5 337
Solar cycle 20 1964-10 14.3 1968-11 157 86 4.1 11.4 285
Solar cycle 21 1976-03 17.8 1979-12 233 111 3.8 10.5 283
Solar cycle 22 1986-09 13.5 1989-11 214 106 3.2 9.9 257
Solar cycle 23 1996-08 11.2 2001-11 180 82 5.3 12.3 619
Solar cycle 24 2008-12 2.2 2014-04 116 49 5.3 In progress 1085 (31 July 2020) - likely final 914
Solar cycle 25 To be determined 1.8 as at December 2019 (Jan 2020: 2.2)
Average 9.0 178.7 4.4 11.04

The following table is instead divided into (unofficial) cycles starting and ending with a maximum, to give a better feel for the number of spotless days associated with each minimum:

Solar Cycles “Start” (Maximum) Spotless days
Solar cycle 10-11 1860-02 406
Solar cycle 11-12 1870-08 1028
Solar cycle 12-13 1883-12 736
Solar cycle 13-14 1894-01 934
Solar cycle 14-15 1906-02 1023
Solar cycle 15-16 1917-08 534
Solar cycle 16-17 1928-04 568
Solar cycle 17-18 1937-04 269
Solar cycle 18-19 1947-05 446
Solar cycle 19-20 1958-03 227
Solar cycle 20-21 1968-11 272
Solar cycle 21-22 1979-12 273
Solar cycle 22-23 1989-10 309
Solar cycle 23-24 2001-11 817
Solar cycle 24-25 2014-04 748 (31 July 2020)

References
  1. "Sunspot numbers". WDC-SILSO. Royal Observatory of Belgium. Retrieved 19 May 2017.
  2. Hathaway, D. (2015). "The solar cycle". Living Reviews in Solar Physics. 12 (1): 4. arXiv:1502.07020. Bibcode:2015LRSP...12....4H. doi:10.1007/lrsp-2015-4. PMC 4841188. PMID 27194958.
  3. Usoskin; Mursula; Kovaltsov (2001). "Heliospheric modulation of cosmic rays and solar activity during the Maunder minimum". J. Geophys. Res. 106(A8) (A8): 16039. Bibcode:2001JGR...10616039U. doi:10.1029/2000JA000105.
  4. Usoskin; et al. (2009). "A solar cycle lost in 1793-1800: Early sunspot observations resolve the old mystery". Astrophys. J. Lett. 700 (2): L154. arXiv:0907.0063. Bibcode:2009ApJ...700L.154U. doi:10.1088/0004-637X/700/2/L154.
  5. "SIDC smoothing formula". WDC-SILSO. Royal Observatory of Belgium. Retrieved 19 May 2017.
  6. Meeus, J. (1958). "Une formule d'adoucissement pour l'activité solaire". Ciel et Terre. 74: 445. Bibcode:1958C&T....74..445M.
  7. "The solar cycle's new clothes". STCE. Royal Observatory of Belgium. Retrieved 8 July 2015.
  8. "What's Wrong with the Sun?". NASA Science. NASA. Retrieved 29 July 2014.
  9. "Spotless Days Page". SILSO. Royal Observatory of Belgium. Retrieved 3 April 2017.
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